This invention relates generally to gas-burning apparatus, such as furnaces, and in particular to a device for reducing NOx emissions in gas-burning apparatus.
The formation of nitrous oxide (NOx) occurs when nitrogen is oxidized during the burning of a gaseous fuel. The higher the flame temperature, the greater the amount of NOx that is produced by the burner flame. Air quality standards in some states (e.g., California) require that NOx emissions from gas furnaces not exceed 40 nanograms per Joule (ng/J).
Devices for reducing NOx emissions in a gas furnace are known in the art. Such devices typically include target or impingement plates, located just inside the combustion chamber in proximity to the furnace burner, for quenching the flame and reducing the temperature thereof. Devices of this type are shown in U.S. Pat. Nos. 5,244,381 and 5,961,320. Another type of NOx reduction device uses a baffle member located in a gas furnace combustion chamber, which enhances mixing between the flame and the secondary combustion air downstream of the baffle member, as shown in U.S. Pat. No 5,472,339.
In accordance with the present invention, a device for lowering NOx emissions from a gas-burning apparatus is provided. The apparatus has a combustion chamber with an inlet and an outlet, a burner proximate to the inlet for burning a combustible mixture of gas and primary combustion air to produce a relatively gas-rich flame, and a blower for drawing the gas-rich flame along with secondary combustion air into and through the combustion chamber. The device includes a perforate member located in the combustion chamber at a predetermined distance from the inlet thereof.
In accordance with a feature of the invention, the perforate member has a generally cylindrical configuration, which extends along the direction of flow of the flame in the combustion chamber a sufficient amount to ensure that the flame makes a plurality of passes through the perforate member as it is drawn through the combustion chamber. By ensuring such plurality of passes, the dwell time of the perforate member in the flame is sufficient to quench the flame and inhibit NOx production.
In accordance with another feature of the invention, the perforate member has a plurality of holes which are sized and spaced apart at sufficient distances to provide sufficient porosity to allow ample mixing of products of combustion, while still providing sufficient flame quenching.
In accordance with yet another feature of the invention, the perforate member is attached to a cup member by means of at least one elongated mounting arm. The cup member is located proximate to the inlet of the combustion chamber and is tapered inwardly toward the perforate member to enhance mixing between the gas-rich flame and the secondary combustion air and to direct the flame down the center of the combustion chamber in the direction of the perforate member.
In accordance with a preferred embodiment of the invention, the perforate member is located in the combustion chamber about two inches inwardly from an innermost edge of the cup member and has a length of about 1.375 inches along the combustion chamber. The perforate member preferably has a diameter of about one inch. Each of the holes in the perforate member preferably has a diameter of about 0.125 inch and the holes are spaced apart at intervals of approximately 0.188 inch.
In operation, the gas-rich flame produced by the burner is drawn into the combustion chamber along with secondary combustion air. The tapered configuration of the cup member mixes the secondary combustion air with the gas-rich flame to provide a relatively compact flame near the center of the combustion chamber. When the flame encounters the perforate member, it makes multiple passes therethrough, whereby turbulence is increased to enhance mixing of the products of combustion. The perforate member also acts as a heat sink to reduce the temperature of the flame and inhibit NOx production.